<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    body{
      margin: 0;
      padding: 0;
      background-image: url(../../images/grid.png);
    }
    canvas{
      background-color: rgba(255, 0, 0, 0.1);
    }
  </style>
</head>
<body>

  <canvas id="tutorial" width="300" height="300px">
    你的浏览器不兼容Canvas,请升级您的浏览器!
  </canvas>

  <script>
    window.onload = function() {
      let canvasEl = document.getElementById('tutorial')
      if(!canvasEl.getContext){
        return
      }
      let ctx = canvasEl.getContext('2d') // 2d | webgl

      let sun = new Image()
      sun.src = '../../images/canvas_sun.png' 
      // sun.onload = function() {
      //   // draw
      // }
      let earth = new Image()
      earth.src = '../../images/canvas_earth.png' 
      let moon = new Image()
      moon.src = '../../images/canvas_moon.png' 


      requestAnimationFrame(draw)

      /**
       1秒钟会回调 61次
      */
      function draw() {
        ctx.clearRect(0, 0, 300, 300)
        ctx.save()
        
        // 1.绘制背景
        ctx.save()
        ctx.drawImage(sun, 0, 0) // 背景图
        ctx.translate(150, 150) // 移动坐标
        ctx.strokeStyle = 'rgba(0, 153, 255, 0.4)'
        ctx.beginPath() // 绘制轨道
        ctx.arc(0, 0, 105, 0, Math.PI * 2)
        ctx.stroke()
        ctx.restore()
 

        ctx.restore()
        requestAnimationFrame(draw)
      }

    }
  </script>
</body>
</html>